1. Field of the Invention
This invention relates to a method for controlling the increase in the molecular weight of nylon 6 during solid phase polymerization while simultaneously reducing the amount of caprolactam and oligomers in the nylon 6.
2. Prior Art
U.S. Pat. No. 4,816,557 discloses a process which involves preheating nylon 6 granules to a temperature of not less than 100 degrees C. and then passing the granules downward through a treatment zone which is heated from 130 to 210 degrees C. A countercurrent of superheated steam is present in the treatment zone. There is no discussion of polymerization and affecting polymerization rates by sequential temperature exposure.
U.S. Pat. No. 5,576,415 discloses a method for drying polyamide and then increasing its molecular weight by solid-phase treatment. The moisture content of the polyamide is regulated at the glass transition temperature of the polyamide, and then the polyamide is heat-treated. The heat treatment involves first heating the polyamide at a pressure of at least one atmosphere, without decreasing the moisture content, for a time sufficient to produce a polyamide with up to 15% crystallinity. Then the polyamide is heated at a temperature lower than the melting point of the polyamide at reduced pressure. No discussion of solid phase polymerization at temperatures below the preheating temperature in order to slow polymerization rate is made.
U.S. Pat. No. 5,596,070 discloses solid state polymerization of polymers at a temperature 5 to 100 degrees C. below the melting point of the polymers, wherein the solid state polymerization occurs in the presence of an inert gas which contains at least 50% by volume of superheated steam. While disclosing temperatures, gas flow rates and compositions, this patent does not disclose using heat treatment to slow molecular weight buildup to allow time for escape of undesirable volatiles.
U.S. Pat. No. 5,859,180 discloses a process for the solid state polycondensation of polyamide resins, wherein ratio of the throughput by weight of an inert gas fed to a reactor and the throughput by weight of the polymer at a reactor outlet is lower than 0.5. Techniques including preheating are disclosed, but the use of preheating then cooling as a means of slowing the rate of molecular weight increase is not disclosed.
U.S. Pat. No. 6,069,228 discloses solid phase polymerization of nylon 6 with simultaneous removal of polyamide precursors and caprolactam. No technique for slowing the relative rate of molecular weight build up relative to extractable removal is taught.
The present invention is a process for making nylon 6 having a desired molecular weight and a desired caprolactam content, comprising
(1) heating nylon 6 having an initial molecular weight less than the desired molecular weight and an initial caprolactam content greater than the desired caprolactam content to a first temperature in the range of 130 to 220 degrees centigrade in the presence of an inert gas;
(2) maintaining said first temperature for a time sufficient to raise the molecular weight of the nylon 6 by 5% to 95% of the difference between said initial molecular weight and the desired molecular weight;
(3) lowering the temperature of said nylon 6 to a second temperature at least 1 degree centigrade below said first temperature; and
(4) maintaining said second temperature for a time sufficient to achieve the desired caprolactam content and the desired molecular weight.